Probably the most often misunderstood element of KDE4 is Plasma, the extensive widget engine that replaces the normal desktop and the Kicker panel from KDE 3.x. The entire KDE4 desktop is built up out of Plasmoids (yet another term for desk accessory), including the panel and the desktop itself - and it is the latter that has been causing quite some confusion. Where are my desktop icons? Update: Aaron Seigo has published a screencast showing how the FolderView Plasmoid behaves as a normal desktop, and how to make it so.

all plasmoids are freely resizable so you can just "maximise it like any other window". Its background is semitransparent, but opacity is dependent on plasma theme, which, for example glassified is more transparent than the default.

Hmm, I didn't realize it was as easy to work around as just putting a giant expanded plasmoid on your desktop. Is it also see-through or does it have background-image capabilities?

You can make a folderview desktop plasmoid see-through, so that the icons in the folder view appear as though they are directly on the desktop, bounded by a ghost of a box (which is in fact the folderview window).

You can have several such boxes on your desktop (they can be folderviews of different folders, or they could be different filters on the same folder). You could, for example, have one box for shortcuts to application executables, another box for "working files", or a couple of boxes for working files for different tasks, and/or you could have a box for "TODO notes". You can size and position (and even rotate) each folderview box wherever you want to on your desktop.

The only constraint that I can see is that you currently can't arbitrarily re-arrange the positions of individual icons within each folderview box ... within each box you are currently constrained to have icons laid out on a grid, and ordered in some way (such as alphabetically).

Here is an example of multiple transparent folderviews placed on a KDE 4 desktop: